Sounding Out with the OLPC XO

In January of this year, I received an XO laptop from the One Laptop Per
Child (OLPC) Project, thanks to a kindly recommendation from my friend
Dr Richard Boulanger, professor of music synthesis at the Berklee
College of Music. Rick knows that I maintain a private teaching studio
and that many of my students are youngsters who would love to play with
the XO. He also knows that I have a twin interest in Csound and Linux
audio development, two rather significant aspects of the machine. Thus,
this article focuses on my experiences so far with the XO's audio
subsystem and its sound and music software. My students have had only
brief exposure to the machine, but I conclude with some remarks
concerning their interaction with the XO and its audio capabilities.

General Overview of the XO

There's plenty of material on the Web that describes the XO in minute
detail, so here I recap only the most salient features of the machine.

The XO laptop (Figure 1) is small and lightweight without feeling flimsy
or poorly constructed, and the few mobile parts are connected
firmly at their joints. The display swings up from the base and can be rotated
180 degrees left or right in its upright position. It also can be tilted
slightly backward. The keyboard is a single rubber membrane, designed for
kid-size fingers, but ham-handed adults like yours truly can plug in a
USB keyboard if necessary. A two-button touchpanel replaces the mouse,
though currently only one panel and one button are active. That's not
a problem, because only the pointer control and an entry button are required to
navigate the GUI.

Figure 1. One Tricked-out XO at Studio Dave

I'm impressed by the thought that has gone into the design of
the XO. At every level, I find consideration for the user's experience,
from the design of its battery pack to the excellence of its display
resolution. In fact, when I've shown the machine to friends, they've all
especially admired the handle and wondered aloud why their laptops didn't
include one.

On the software side, the XO is powered by a modified version of
Fedora Core with a 2.6.22 Linux kernel. The GUI is the renowned Sugar,
a Python-based graphic interface that is singularly unlike the typical
Linux desktops with which I'm familiar, and the Linux command-line is
easily available at any time.

Audio Hardware and Capabilities

The XO's CPU is a 433MHz AMD Geode LX-700. The laptop's multimedia
capabilities are provided by the Geode CS5535/CS5536 companion
chipset. According to the Wikipedia page on the Geode, the CS5535 is a
“...Southbridge for Geode GX and Geode LX...[that] integrates four
USB ports, one ATA-66 UDMA controller, one infrared communication port,
one AC97 controller, one SMBUS controller, one LPC port, as well as GPIO,
Power Management, and legacy functional blocks”. The processor's AC97
controller is of central importance to this article, along with the
possibilities afforded by the USB support, so let's consider exactly what
that AC97 is and what it does.

In 1997, Intel developed an audio codec to provide high-quality audio
services for motherboards, modems and sound hardware. The AC97 defines
a high-quality audio architecture with a sampling rate of up to 96kHz for stereo and 48kHz for multichannel digital audio recording and
playback, with bit depths up to 20 bits. The AC97 became very popular with
manufacturers and is found on most desktop machines, though it has been
superseded recently by Intel's HDA (high-definition audio). The codec
is divided into a digital controller and an analog stream handler,
effectively combining the analog-to-digital and digital-to-analog
converters in a single package (an appealing feature for hardware
designers). By the way, Intel's use of the word codec here refers to the
encoding/decoding of analog-to-digital and digital-to-analog streams,
as distinct from binary compression/decompression codecs such as MP3, Ogg
or WMA/WMV.

The AC97 implementation for the CS5535 comes from an integrated Analog
Devices AD1888 chipset that provides up to six channels of digital or
analog audio output. The AD1888 is notable also for its direct connection
to the core CPU, a cost-saving factor that accords nicely with the
XO's overall design. The XO also uses another Analog Devices chipset
(the SSM2211) for audio amplification.

So much for audio on the inside. On the outside, we find an integrated
microphone, two integrated speakers and jacks for stereo audio output
(to headphones or other speakers) and for a monaural microphone-level
input. The jacks are standard consumer-grade sound-card connectors that
take 3.5mm mini-plugs, and I'm happy to report that connections to those
jacks are firm and steady. The jack functions also are redefinable with
the alsamixer utility, but I did not experiment with this feature. See
the OLPC Wiki page on the XO's audio hardware for more information about
redefining the audio I/O ports.

The XO also includes three USB ports. Obviously, these ports
can be used to expand the machine's audio capabilities by adding a MIDI
interface or a higher-quality digital audio interface.

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